Method of and system for dissipating the heat generated by electronic control devices in cryogenic installations

ABSTRACT

A method of and a system for dissipating the heat produced by electronic components, especially motor-control thyristors (silicon-controlled rectifiers) used in motor-control circuits in cryogenic installations. A part of the cooled fluid circulated in the installation is diverted from its circulation or displacement path and used to cool the power thyristors which are employed in frequency inverters and like electronic circuitry used to control the speeds of compressor motors of such installations. The thyristor may be mounted in vaporizer or evaporator of the cryogenic installation which preferably is a refrigeration unit or system for the low-temperature rectification of air or other gas mixtures.

United States Patent Henrici [151 3,656,540 [451 Apr, w, 1972? [54] METHOD OF AND SYSTEM F OR DISSIPATING THE HEAT GENERATED BY ELECTRONIC CONTROL DEVICES IN CRYOGENIC INSTALLATIONS [72] Inventor: Helmut Henrici, Rodenkirchen, Germany [73] Assignee: Linde Aktiengesellschaft,

skreuth, Germany [22] Filed: Nov. 18,1969

[21] App]. No.: 877,761

[30] Foreign Application Priority Data Nov. 18, 1969 Germany ..P 18 09 770.6

[52] US. Cl ..165/1, 165/29, 62/498,

317/234 [51] Int. Cl ..FZSb 29/00 [58] Field of Search 165/29, 1, 2; 62/498; 317/234 l-lollriegel- [56] References Cited UNITED STATES PATENTS 3,330,131 7/ 1967 Papst ..62/498 3,400,543 9/1968 Ross ..317/234 3,476,175 11/1969 Plevyak ..317/234 Primary Examiner-Charles Sukalo Attorney-Karl F. Ross [57] ABSTRACT A method of and a system for dissipating the heat produced by electronic components, especially motor-control thyristors (silicon-controlled rectifiers) used in motor-control circuits in cryogenic installations. A part of the cooled fluid circulated in the installation is diverted from its circulation or displacement path and used to cool the power thyristors which are employed in frequency inverters and like electronic circuitry used to control the speeds of compressor motors of such installations. The thyristor may be mounted in vaporizer or evaporator of the cryogenic installation which preferably is a refrigeration unit or system for the low-temperature rectification of air or other gas mixtures.

12 Claims, fl Drawing Figures FROM CRYOGENIC CIRCUIT PATENTEDAPR 18 I972 3, 656, 54 O sum 2 OF 2 Helmu'r Henrici INVI5N'IY1R.

' Attorney METHOD OF AND SYSTEM FOR DISSIPATING THE HEAT GENERATED BY ELECTRONIC CONTROL DEVICES IN CRYOGENIC INSTALLATIONS l. FIELD OF THE INVENTION The present invention relates to a method of and a system for dissipating the thermal energy of electronic motor-control devices used in cryogenic installations and, more particularly, to a system for dissipating the heat involved as a result of heat losses in thyristors (solid-state or silicon controlled rectifiers) employed in variable-frequency inverter circuitry for controlling the speed of alternating-current motors used in driving the compressors of the cryogenic or refrigeration installation.

2. BACKGROUND OF THE INVENTION In cryogenic installations in which low-temperature fluids are produced, used or modified, it is a common practice to provide compressors and the like which are driven by alternating-current motors. Similarly, refrigerators, air conditioners, freezers and industrial-cooling systems are known in which motor-driven compressors increase the pressure in a gas while causing the temperature thereof to rise, the fluid under pressure being thereafter cooled (liquefied) and subjected to evaporation, vaporization or expansion which, in accordance with the Joule-Thompson effect, substantially reduces the temperature in the expanded fluid. The cold fluid may then be used for substantially any cryogenic purpose.

In industrial refrigerating and air-conditioning systems, for example, the low-temperature expanded fluid is passed in heat-exchanging relationship with ambient air to lower the temperature of the latter or is applied to a surface to chill the same or in heat-exchanging relationship with another fluid in heat-exchanging relationship with the system to be cooled, the latter coolant is circulated by pumps or the like in a secondary or liquid-coolant cycle.

In cryogenic installations for the separation of gas fractions by low-temperature distillations, the expanded fluid may be cooled to the point of liquefaction, upon passage from the expansion nozzle and may flow in countercurrent to a refluxing gas in a Linde-Friinkl column or other fractionating assembly in which the gas mixture is separated into its components. In the air-rectification installation, for example, air at ambient temperatures may be liquefied and distilled to separate it into its components, e.g. oxygen and nitrogen. In installations of the latter type, compressors are also employed preliminarily to an expansion of the gas.

In all low-temperature installations of the aforedescn'bed type, i.e. refrigerating units as well as the much lower temperature cryogenic gas separators, it is a common practice to provide asynchronous motors for driving the compressors in accordance with the degree of efiiciency of the refrigerating circuit which may be desired. It is, of course, important to control the rate at which the fluid is compressed as well as the degree of compression and, to this end, speed controls have been provided for such motors.

Until the advent of electronic circuitry capable of eliminating the disadvantages of the monofrequency line source, the use of synchronous motors controlled by the supply frequency was accompanied by an inability to effectively vary the rotary speed of the compressor, especially since the desired variation range far exceeded the capability of speed variation with a given input frequency. When compressors are discussed herein, it will be understood that substantially all forms of compressors which may have been used heretofore in refrigerator units and cryogenic installations are contemlated.

p To provide the speed variation of the compressor which was found to be desirable, it has hitherto been the practice to install between the motor and the compressor a variable-speed transmission or to provide the drive motor as a rotary field high-frequency motor which, in turn, was driven by a separately provided high-frequency source. These arrangements had the obvious disadvantages that they were relatively complicated and of high cost.

To eliminate these disadvantages and to obtain similar results at low cost, it has been proposed heretofore to provide speed regulation with the use of electronic switching devices and electronic circuitry incorporating same. Thus, for example, there have been proposed electronic frequency inverters capable of matching the input and line frequency to the desired frequency and phase relationship at a synchronous or like motor driven at a speed not compatible with that associated with the line frequency. In other words, the motor may be driven at speeds characterized by frequencies substantially different from the 50 or 60 cycle supply frequency.

While numerous frequency-inverter motor-control circuits have been provided heretofore, many of them have in common the use of solid-state or silicon controlled rectifiers, (thyristors) which may be described quite generally as threeelement triggerable switching devices having a rectifying characteristic. If the anode and cathode of the thyristor is connected between a source and the load, the gate may be triggered to provide a load current at the frequency of triggering of the gate.

A typical motor-control circuit capable of use with compressor motors, for example, made use of a rectifying network connected to the line source and producing, in efiect, a direct current. A DC to AC inverter is connected to the network and includes thyristor devices triggered at an adjustable frequency to produce a pulsating output which, in turn, drives the motor at a speed determined by the output frequency. These thyristors or silicon-controlled rectifiers are characterized by thermal losses and heat evolution proportional to the current passing through them and therefore the load current. It has also been the practice to provide the thyristors with heat sinks and other cooling devices in the form of fins and the like to increase the ability of the solid-state devices to dissipate heat into the ambient atmosphere at the room temperature at which the surrounding fluid was maintained.

This cooling technique has the significant disadvantage that heat transfer to the ambient air is inefficient at best and requires a configuration and mounting of the thyristor which is inconvenient, awkward and bulky. Furthermore, air cooling requires special fans and is incapable of adequately responding to sudden changes in the heat evolution of the thyristor to the point that overloading of this electronic component and failure was not uncommon.

3. OBJECTS OF THE INVENTION It is, therefore, the principal object of the present invention to provide an improved system for cooling electronic devices 4. SUMMARY OF THE INVENTION These objects and others, which will become apparent hereinafter, are attained in accordance with the present invention, with a system in which a cooling fluid or a cooled fluid,

well below ambient temperatures, is displaced either for separation of the fluid into its components, or to provide cooling elsewhere, or even as a consequence of a separation of a mixture into its components by cryogenic techniques, and where a compressor of the turbine, screw or other type is driven by an electric motor operated with variable speed by a thyristor frequency-changer or phase controller having a frequency inverter in which the thyristors conduct the load or motor current. The present invention provides that the thermal-loss heat of the thyristors is dissipated by a cooling fluid as used in the cryogenic installation, preferably at temperatures below ambient.

The invention is based upon the discovery that the fluids used in cryogenic installations of the type described earlier, when below ambient temperature and at one of the relatively lowcryogenic temperatures maintained in such installations, is able of entering into heat exchange with the thyristors of such an improved nature as to permit the thyristors to operate well above their normal limits in terms of current-carrying capacity.

Consequently, the efficiency and load-carrying capacity of each thyristor is markedly increased so that, for a given load current, a reduced number of thyristors may be employed or thyristors may be used which have a lower capacity per unit. There is, consequently, a saving in cost, which saving is augmented by the fact that the cooling apparatus may be of reduced size with nevertheless increased effectiveness by comparison with conventional thyristor assemblies using ambient air coolers.

In fact, the normal ratings of the thyristor may be ignored and the electronic switching device employed as if its rating was increased severalfold in terms of load-carrying capacity. A given thyristor installation may, accordingly, be used to carry much higher currents than would be the case when the same unit is employed with ambient air cooler.

Of considerable significance is the fact that removal of the thermal energy resulting from heat losses in the thyristor at low temperatures increases the current density which may be sustained by the thyristor circuit so that still higher capacities may be achieved than would otherwise be expected. Preferably, the thyristors are operated at temperatures of about 30 C., and up to say C., the lower limit being the apparent lower limit of the temperature at which present-day thyristors are effective, much lower temperatures adversely affecting the electronic characteristics of the devices. Still lower temperatures may be used with appropriate thyristors.

The present invention is applicable to all thyristor-controlled speed-regulating circuits for the electric motors used in low-temperature installations, including refrigerating units, cryogenic plants and the like and is especially applicable to the frequency inverter controls used for the electric motors driving turbine or screw compressors for such installations. The term cryogenic installation is thus used in the generic and broad sense indicated.

Typical adjustable frequency inverters for variable speed drives with which the present invention is usable, are described by Bradley et al. Adjustible-Frequency Inverters and Their Application to Variable-Speed Drives, Proc. IEEE, (London), Vol. III, pages 1,833 to 1,846, Nov. 1964; the General Electric SCR Manual 4th Edition, General Electric Company, Syracuse, New York, 1967, and the RCA Power Circuits Manual published by the Radio Corporation of America, Harrison, New Jersey (1969).

According to a more specific feature of this invention, the temperature of the thyristor bank and/or the individual thyristor is monitored automatically and the flow of cool fluid to the heat sink of the thyristor or thyristors regulated accordingly to maintain a correlation between the degree of cooling and the load. The control system may also make use of load current detectors for regulating the degree of cooling. With the low-temperature fluids contemplated by the present invention, it is possible for the first time to accurately monitor and adjust the degree of cooling to the load upon the thyristor.

Still another feature of this invention resides in the cooling of the thyristors with a portion of the cooling fluid, derived from the liquid/vapor refrigerant cycle and the secondary heat-transfer fluid (e.g. water or brine), branched from the main circulation and returned after use as a thyristor coolant. The volume rate of flow of the branched stream may be adjusted in relation to the remainder of the circulating fluid in dependence upon the thyristor temperature. In this fashion, a minor fraction, preferably 2 to l0 percent, of the circulating cold fluid is used to cool the thyristors.

Furthermore, it has been found to be advantageous to use as the branch stream, a coolant which may be provided for the cooling of other apparatus in the installation, e.g. the electric motor driving the compressor. In this case, the coolant may be brought to a slightly higher temperature by passage through the motor jacket or may be led to the jacket at a slightly higher temperature. By the adjustment of valves or the like to control the volume rate of flow through the branch stream, it is possible to provide rapid compensation for the very large and rapid load current changes in the energization circuit for the compressor motor to the extent that in practice current changes of to 3,000 amp. have been encountered while the thyristor temperature has been held constant.

Still another feature of this invention resides in the use of a branch stream for thyristor cooling which serves to cool the motor and is so arranged that, during the initial period of operation when motor cooling is unnecessary, the entire branch flow of the coolant passes through the thyristor-cooling assembly, the electronic devices of which may be sustaining maximum current (e.g. at start of the motor) whereas the major portion or at least a considerable proportion of the coolant flow is diverted through the motor and thereafter to the thyristor cooling bank after a period of use at which time the heat loss in the thyristors may be reduced as the heat evolution in the motors increases.

It has been found to be advantageous, moreover, to provide the thyristor flow as a branch stream of the refrigerant circulated in a refrigerating installation from which the coolant is withdrawn from the condenser or a coolant reservoir and is sprayed onto the heat sink of the thyristor or pump through the heat sink thereof. The fluid may then vaporize in the heat sink so that not only its sensible cold (ability to absorb sensible heat), but also its latent condensation cold (equivalent to the heat of vaporization) may be used to absorb heat from the thyristors. In the latter case, proportionately little coolant is required to maintain a constant temperature at the thyristor bank. Moreover, all or part of the frequency inverter, but at least the thyristors thereof can be build into the evaporator stages of a cryogenic installation, in which case part or all of the piping which would otherwise be required to cool the thyristors, can be eliminated.

5 DESCRIPTION OF THE DRAWING The above'and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a block diagram partly in flow-diagram form, of a cryogenic installation for the separation of gases and embodying the principles of the present invention;

FIG. 2 is a cross-sectional view through a thyristor assembly according to the invention, with related parts illustrated in diagrammatic form;

FIG. 3 is a diagrammatic vertical section through a separating column of the cryogenic type; and

FIG. 4 is a diagram of an industrial refrigeration unit according to the invention.

6. SPECIFIC DESCRIPTION In FIG. 1 of the drawing, there is shown a portion of an airrectification installation 10 which may be of the Linde-Frankl type for the separation of air into its principal components, namely oxygen and nitrogen. A typical air-rectification installation of this nature is described at pages 12-26 ff. of Perrys Chemical Engineers Handbook, Fourth Edition, McGraw-Hill Book Company, New York 1963).

The rectification column is shown at 11 with only some of the related apparatus. In general, an incoming air mixture (or other gas mixture to be separated by low-temperature fractional distillation) is drawn into the system through a heat exchanger 12 from an inlet 13 and subjected to partial cooling by one of the cool products derived from the gas-rectification tower 11. From the initial cooling stage 12, the gas mixture is fed to a compressor 14, here shown to be driven by an electric motor which may have variable speed in accordance with the degree of compression and the quantity of gas fed into the system.

According to the principles of this invention, therefore, the frequency determining the speed of the motor 15 may be established by an adjustable DC-to-AC inverter 16 of the thyristor type in which one or more power thyristors represented at 17, is triggered to render the inverter conductive and pass the load current to the motor 15.

DC-to-AC inverters of this type are described at pages I62 ff. of Power Circuits, published by the Radio Corporation of America, Harrison, New Jersey (1969) and at pages 225-257 of the SCR Manual, Fourth Edition, published by the General Electric Company, Syracuse, New York (1967). Such circuits may receive power from the AC line source 18 which can operate at 50 or 60 Hz. via a phase-controlled rectifier 19, the output of which is established by a feedback loop represented at 20. The thyristor or controlled-rectifier circuit for operating the motor 15 is, to the extent described, entirely conventional.

It has been noted previously, however, that there are thermal losses in the variable frequency inverter circuits used in controlling motors such as that shown at 15 for the compressor 14 of the cryogenic installation and that these losses are related to the current flow through the thyristor or silicon controlled rectifier or the bank of thyristors.

In place of the normal atmospheric dissipation of the heat energy evolved by the thyristors, the present invention provides a cooling of the thyristors by bypassing a portion of the cold fluid circulated or used in the cryogenic installation, to the thyristors as represented by the branch line 21 and a valve 22. The latter taps a portion of the compressed, but nevertheless cool gas (by comparison with ambient temperature) from the turbine compressor 14.

It is, however, also possible to make use of part of the cold gas after it has been produced by expansion in an evaporator 23 as represented by the valve 24.

In other alternatives, the cold fluid derived from the initial heat exchanger 12 may be employed (valve 25) or use may be made of the cold separated fraction recovered at 26 from the heat exchanger 12 and derived from the rectification tower l 1.

A valve 27 is provided to deliver part of the cold separated gas to the thyristors 17 of the frequency inverter or motorcontrolled circuits 16-20. After being used to cool the thyristors, the gas may be returned to an appropriate point in the fluid circuit as represented by the branch 28 which leads into the rectification column with the gases formed by expansion at 23.

Normally, the expansion and vaporization of the output of the compressor 14 will be carried out within the rectification column, as shown for the column 30 in FIG. 3. In this column, the fluid derived from the compressor is passed through a coil 31 in the reflux boiler 32 at the base of the column 30 to vaporize previously liquified gas represented as a pool 33. The gases pass upwardly (arrow A) through a multiplicity of fractionation stages 34, countercurrent to a descending trickle of liquid cascading from the fractionation stages or trays 34. After traversing the boiler, the compressed fluid at a substantially lower temperature is led to a nozzle 35 from which expanding fluid is dispensed, at least a portion of the expanding fluid being liquified in accordance with the principles involved in the well-known Joule-Thomson effect. Auxiliary cooling may be provided at 36 to produce a further cascade of liquified gas droplets. The thyristors of the frequency control, speed-regulating circuit for the motor 15, may be introduced directly into the evaporator or vaporizing unit as will be apparent from FIG. 4 or else incorporated into some other chamber maintained at low temperature, e.g. the boiler 32 of the rectification column 30. In FIG. 3, the thyristors are shown at 38 to be enclosed in tube 39 immersed in the liquid 33 of this column.

In FIG. 2 there is shown an arrangement, in accordance with this invention, which is applicable to substantially any low temperature or cryogenic circuit including the cryogenic installation of FIG. 2 and the refrigerating circuit of FIG. 4. In this system, the motor 41 is shown to be provided with a jacket 42 through which a cooling fluid may be led. The motor 41 drives a turbine or screw compressor 43 of the refrigerant or vapor/liquid cycle of the cryogenic circuit which is not otherwise illustrated but may be the same as that of FIG. 1 or of FIG. 4.

In the system of FIG. 2, the refrigerant of the vapor-liquid cycle, a secondary coolant such as water or brine, or a cold gas product of air rectification or a gas mixture adapted to be rectified, constitutes the circulated coolant passing through the cryogenic circuit branch 44. A valve 45 in this line, serves to divert a portion of the coolant via a conduit 46 to the motor jacket 42 in a motor-cooling stream. Thereafter, the coolant at temperatures well below ambient, is passed via line 47 to the heat sink 48 of a bank of thyristors 49 connected in a frequency-inverter circuit as diagramatically represented at 50 for controlling the speed of the motor. The heat sink 48 is formed with a duct 51 through which a coolant may be circulated via a pump 52 and returned to line 44 through a check valve.

The valve 54 serves to divert all of the branched coolant to the thyristor housing 48 via line 55 and a check valve 56 during the initial period of operation-of the motor, when maximum heat loss is sustained by the thyristors 49 but minimal heat evolution occurs at the motor 41. Gonversely, during later periods of operation'the heat evolved at the motor increases while the heat loss at the thyristors drops whereupon the temperature controller 57 shifts valve 54 to pass all of the branched coolant through the jacket 42 of the motor 41.

The thermostatic device 57 has as its input a plurality of thermocouples soddered at 58 to the bases 58 of the thyristors 49 and, in turn, regulates the valves 45 and 54 to control the volume rate of flow of the branched cooler and maintain the thyristor temperature substantially constant in spite of variations in the heat loss in the thyristors resulting from fluctuations in the load current.

In FIG. 4, the invention is illustrated as it applied to a refrigerating installation of the industrial type. In this installa tion, the motor 60 drives a compressor 61 connected in the liquid-vapor refrigerant cycle which includes a heat dissipator 62 to which the heat built up upon compression of the refrigerant and liquefaction thereof is dispelled into the atmosphere via, for example, a fan 63. The fan, however, merely represents any conventional means for dissipating the heat of liquefaction and is, of course, equivalent to circulating water systems having cooling towers or the like. The refrigerant cycle includes an expansion valve 64 leading into the evaporator heat exchanger 65 through which brine or water is circulated via a pump 66 as part of a secondary coolant circuit. From the heat exchanger 65, the refrigerant is returned to the compressor via a line 67 to complete the refrigerant cycle.

A secondary coolant circuit includes a load 68 which may be cooled by the circulating water or brine and from which a warmed liquid is returned to the pump 66 via a line 69.

As illustrated in FIG. 2, the motor 60 is of the altematingcurrent type whose speed may be regulated by a variable frequency inverter represented diagrammatically at 70 and energized by the rectifier network 71 from the alternating-current line source 72. The thyristor inverter 70 is here shown to have its thyristors 73 mounted in an evaporator housing 74 to which a branch 75 of the refrigerant-circulating path leads. A valve 76 determines the proportion of refrigerant bypassed to the housing 74 into which the refrigerant is separated via the expansion valve 77 to vaporize within the housing 74 in heat exchanging relationship with the thyristors. The additional cooling provided by this vaporization and absorption of the latent heat of vaporization, permits the thyristors to be cooled with a minimum of refrigerant. The valve 76, controlling the bypassed refrigerant flow rate, is regulated by a temperature or load-current sensor 78 having as its inputs, a thermocouple 79 connected to the thyristors or a current-sensing ammeter 80 connected in the load circuit.

One or more of the thyristors of the frequency inverter may be cooled as represented at 81. In this arrangement, a portion of the secondary coolant displaced by pump 66 is drawn from the secondary circuit after the heat exchanger 65 and is led to the thyristor housing as shown at 81 and thence, via a check valve 82 through the housing 83 of the motor 60 before being returned to the secondary coolant circuit by a line 84. A pair of valves 85 and 86, controlled by a thermostat 87 responsive to the temperature of the thyristors 81, split the bypassed coolant flow between the motor and the thyristor housing to maintain the thyristors at a constant temperature.

A plurality of thyristors 90, 91 and 92 of the variable frequency inverter may be provided with respective housings through which portions of the secondary coolant may be individually bypassed via thermostatically controlled valve 93 individually associated with these thyristor housings and regulated by a thermostatic device 94 responsive to the individual thyristor temperatures. Upon emerging from these housings, the secondary coolant is returned via a check valve 95 to line 69.

The invention as illustrated and described is believed to admit of many modifications which will be obvious to those skilled in the art and which are intended to be encompassed within the scope of the appended claims.

lclaim:

l. A method of operating a cryogenic installation comprising a vapor/liquid refrigerant cycle including a compressor an electric motor coupled with said cycle for driving same and a frequency inverter circuit having at least one thyristor carrying load current of the motor, said refrigerant cycle including means forming a recirculation path for a refrigerant at a temperature below ambient, said method comprising the steps of cooling said thyristor by diverting a portion of said refrigerant from said path into heat-exchanging relationship with said thyristors returning said portion to said path upon cooling of said thyristors and adjusting the proportion of refrigerant diverted from said path in response to the temperature of the thyristor to maintain said temperature substantially constant.

2. The method defined in claim 1 wherein said cycle includes a cold-water circulation path, said thyristor being mounted on a heat sink formed in said cold-water path with a passage, said fluid being cold water branched from said coldwater path.

3. The method defined in claim 1 wherein said portion of said refrigerant is passed in heat-exchanging relationship with said motor for cooling same.

4. The method defined in claim 1 wherein said refrigerant is alternately vaporized and liquefied in said vapor/liquid cycle, said refrigerant being passed in heat-exchanging relationship with said thyristor by vaporizing said refrigerant from the liquid state in a passage formed in a heat sink for said thyristor.

5. A cryogenic installation comprising an electric motor forming part of a vapor/liquid refrigerant fluid displacement system having a circulation path for at least one recirculated cool fluid at a temperature below ambient temperature, a variable-frequency inverter connected to said motor and having power thyristors carrying the load current of said motor, means for branching a portion of said fluid from said path and passing said portion into heat-exchanging relationship with said thyristors, and means responsive to the temperature of said thyristors for controlling the proportion of said fluid branched from said path to form said portion and to maintain said temperature substantially constant.

6. The installation defined in claim 5 wherein said thyristors are mounted upon at least one heat sink provided with a fluid passage, said portion of said cool fluid being passed through said passage.

7. The installation defined in claim 6, comprising a liquid/vapor refrigerant cycle including a compressor driven by said motor for alternately liquefying and vaporizing a refrigerant upon circulation through said cycle, said refriglerant constituting said fluid.

8. he installation efined in claim 7 wherein said heat sink forms part of an evaporator connected in said cycle.

9. The installation defined in claim 7, further comprising means for vaporizing liquid refrigerant of said cycle into said passage.

10. The installation defined in claim 5, further comprising valve means connected in said path and controlled by said means responsive to temperature for regulating the proportion of said refrigerant branched through said passage.

11. The installation defined in claim 7, further comprising a secondary coolant cycle in heat-exchanging relationship with said refrigerant and including a liquid coolant circulation path provided with a pump for displacing said liquid coolant, said liquid coolant constituting said fluid, said liquid coolant path including valve means controlled by said temperature-sensitive mans for branching a portion of said liquid coolant through said passage.

12. The installation defined in claim 11 further comprising means for conducting at least part of said cool fluid in heatexchanging relationship with said motor to cool the same. 

1. A method of operating a cryogenic installation comprising a vapor/liquid refrigerant cycle including a compressor an electric mOtor coupled with said cycle for driving same and a frequency inverter circuit having at least one thyristor carrying load current of the motor, said refrigerant cycle including means forming a recirculation path for a refrigerant at a temperature below ambient, said method comprising the steps of cooling said thyristor by diverting a portion of said refrigerant from said path into heat-exchanging relationship with said thyristors returning said portion to said path upon cooling of said thyristors and adjusting the proportion of refrigerant diverted from said path in response to the temperature of the thyristor to maintain said temperature substantially constant.
 2. The method defined in claim 1 wherein said cycle includes a cold-water circulation path, said thyristor being mounted on a heat sink formed in said cold-water path with a passage, said fluid being cold water branched from said cold-water path.
 3. The method defined in claim 1 wherein said portion of said refrigerant is passed in heat-exchanging relationship with said motor for cooling same.
 4. The method defined in claim 1 wherein said refrigerant is alternately vaporized and liquefied in said vapor/liquid cycle, said refrigerant being passed in heat-exchanging relationship with said thyristor by vaporizing said refrigerant from the liquid state in a passage formed in a heat sink for said thyristor.
 5. A cryogenic installation comprising an electric motor forming part of a vapor/liquid refrigerant fluid displacement system having a circulation path for at least one recirculated cool fluid at a temperature below ambient temperature, a variable-frequency inverter connected to said motor and having power thyristors carrying the load current of said motor, means for branching a portion of said fluid from said path and passing said portion into heat-exchanging relationship with said thyristors, and means responsive to the temperature of said thyristors for controlling the proportion of said fluid branched from said path to form said portion and to maintain said temperature substantially constant.
 6. The installation defined in claim 5 wherein said thyristors are mounted upon at least one heat sink provided with a fluid passage, said portion of said cool fluid being passed through said passage.
 7. The installation defined in claim 6, comprising a liquid/vapor refrigerant cycle including a compressor driven by said motor for alternately liquefying and vaporizing a refrigerant upon circulation through said cycle, said refrigerant constituting said fluid.
 8. The installation defined in claim 7 wherein said heat sink forms part of an evaporator connected in said cycle.
 9. The installation defined in claim 7, further comprising means for vaporizing liquid refrigerant of said cycle into said passage.
 10. The installation defined in claim 5, further comprising valve means connected in said path and controlled by said means responsive to temperature for regulating the proportion of said refrigerant branched through said passage.
 11. The installation defined in claim 7, further comprising a secondary coolant cycle in heat-exchanging relationship with said refrigerant and including a liquid coolant circulation path provided with a pump for displacing said liquid coolant, said liquid coolant constituting said fluid, said liquid coolant path including valve means controlled by said temperature-sensitive mans for branching a portion of said liquid coolant through said passage.
 12. The installation defined in claim 11 further comprising means for conducting at least part of said cool fluid in heat-exchanging relationship with said motor to cool the same. 